The invention relates to the field of X-ray radiography, in particular to an X-ray apparatus with an X-ray source for producing a beam of X-ray radiation, an X-ray detector to detect the X-ray radiation and a filter comprising filter elements containing an X-ray absorbing medium and positioned between the X-ray source and the X-ray detector to absorb the X-ray radiation, in which the X-ray absorption properties of filter elements are individually controllable by changing a length of a column of the X-ray absorbing medium in a propagation direction of the X-ray beam, electrical means being provided, comprising a voltage supply means for the filter elements and an electrical measuring means to measure a quantity which is related to the X-ray absorption properties of individual filter elements.
An X-ray apparatus of this type provided with an X-ray equalization filter to be positioned between the X-ray source and the X-ray detector is known from American U.S. Pat No. 5,778,046. The known X-ray equalization filter comprises an X-ray absorbing mask housed within a shapeable membrane which is positioned in the primary X-ray beam, wherein the local thickness of the mask material can be varied. The known X-ray apparatus can be used in the field of radiodiagnostics, in particular to obtain X-ray transmission images of human body. It is well known that some regions of human body, in particular thorax, have structures of quite different X-ray absorption, which result in pixels on the X-ray detector of different blackening in the X-ray transmission image. The range of the blackening in one X-ray transmission image is defined as dynamic range. The function of the X-ray equalization filter is to compensate the patient regions of low electron density, matching the dynamic range of an X-ray imager in order to obtain a better quality of the X-ray transmission image.
In the known X-ray apparatus a matrix of mechanical pistons is used which can locally modify the length of the column of the X-ray absorbing mask and by doing this, perform a local alteration of the X-ray fluence. The pistons are driven by nickel-titanium wires, wherein the individual length of each nickel-titanium wire is controlled by an electric current applied to the wire. When the electric current is applied to the wire, the wire temperature increases and the wire length decreases. Therefore, there is a certain time necessary for a nickel-titanium wire to shrink or to resume an original length. Further, according to the logistics of the known X-ray apparatus, first a low exposure X-ray image is obtained to calculate the necessary mask profile, then the shaped mask is positioned in the primary X-ray beam. Therefore, there is a considerable time lost for the whole procedure. In the known X-ray apparatus the desired two-dimensional equalization profile of the X-ray absorbing mask is obtained within five seconds. In the field of radiodiagnostics the timing properties of the image acquisition play an important role, especially due to the image artifacts resulting from the movements of the thorax.
Further, the known X-ray equalization filter comprises a limited number of pistons, which is in the order of 8xc3x978 or 16xc3x9716. However, for a high quality X-ray diagnostic image this degree of spatial resolution may not be sufficient. Next to this, for the quality assurance purposes and operation control of the X-ray equalization filter it is desirable to have means of a local length detection of the column of the X-ray absorption medium within the filter. In the known X-ray apparatus the position of each piston is sensed during potentiometers which are specifically constructed for that purpose. Due to the movement of the piston with respect to the potentiometer the local resistance in the corresponding electric circuit is varied, whereby it is assumed that the degree of the resistance variation has a direct correlation with the length of the column of the X-ray absorption mask.
It is an object of the present invention to provide an X-ray apparatus with a dynamic beam attenuation filter with a high degree of spatial resolution and a high operation speed, where the length of column the X-ray absorption medium is directly sensed using a simple electrical circuit. This is achieved in the X-ray apparatus according to the invention, which is characterized in that the filter elements are tube-like structures with an inner volume for containing the X-ray absorbing medium being comprised in a fluid filling having electrically conducting and X-ray absorbing properties, the X-ray absorption of each filter element depending on the length of the column of the X-ray absorbing fluid in the inner volume, each filter element comprising a first electrode for supplying a first electric potential to its wall and a second electrode for supplying a second electric potential to the electrically conducting fluid, the first electrode being separated from the electrically conducting fluid by means of an isolator layer so that an electric capacitance is defined between the first electrode and the electrically conducting fluid, the electrical measuring means being further embodied to measure the electric capacitance in individual filter elements.
In the X-ray apparatus according to the invention the high degree of spatial resolution is achieved by constructing a filter comprising a plurality of tube-like filter elements assembled together, each individual filter element containing an X-ray absorption fluid with an individually controllable level of the X-ray absorption fluid within each filter element, the actual level of the X-ray absorption fluid within the filter element being sensed using simple electrical means. The operation of each filter element is based on the known physical effect of electrowetting.
To explain the operation of the filter element different compositions of the X-ray absorption fluid are to be considered. Firstly, the X-ray absorption fluid with electric conduction and X-ray absorption properties can be obtained by dissolving a salt into a liquid (for example, water). The resulting solution is a single component fluid with both electric conducting and X-ray absorbing properties. Apart from this, it is possible to produce a two-component fluid solution, the first fluid component having exclusively electric conducting properties and a negligible degree of X-ray absorption, the second fluid component having exclusively X-ray absorption properties and being an insulator. The resulting solution is a fluid with both electric conducting and X-ray absorbing properties. It is also possible to produce a two-component fluid from two mutually not mixable fluids, the first fluid having mainly electric conducting properties and a negligible degree of X-ray absorption, and the second fluid having mainly X-ray absorption properties and being an insulator. The resulting fluid will contain two fluid columns on top of each other with the corresponding physical properties attributed to a corresponding fluid component.
The essence of the phenomenon of electrowetting is the variation of the contact angle between the electrically conducting fluid on top of the metal and the metal and thus, the variation in the hydrophobicity of the contact surface. For that purpose each filter element comprises a first electrode located in the wall of the filter element in order to bring an electric potential to the wall. A second electrode is located in the inner volume of each filter element in order to bring a second electric potential to the electrically conducting fluid. The first electrode in separated from the inner wall by means of an isolator layer. For a given composition of the filter element there is a set of operation voltages, namely a xe2x80x98fillxe2x80x99 voltage to fill the filter element with the electrically conducting fluid, a xe2x80x98holdxe2x80x99 voltage to hold the meniscus of the electrically conducting fluid on the desired level and an xe2x80x98emptyxe2x80x990 voltage to initiate the emptying process of the filter element. The operation of the filter element of this type is known and is described in more detail in American U.S. Pat. No. 5,625,665 (PHN 15044). In case the X-ray absorption medium is a single component fluid or a solution of the electrically conducting fluid component and the X-ray absorbing fluid component, there is a single liquid column within the filter element, the level of which defines the degree of X-ray absorption. In the case the X-ray absorbing fluid has more than one, for example two not mutually mixing components, there are two separate fluid columns in the inner volume of the filter element in the X-ray propagation direction, having a mutual separation plane perpendicular to the X-ray propagation direction, whereas the length of the column of the X-ray absorbing fluid component within the filter element and, thus, the degree of the X-ray absorption is passively defined by the length of the column of the electrically conducting fluid component.
Due to the fact that the first electrode is separated from the electrically conducting fluid component by the isolator layer, an electric capacitance is created. The value of the electric capacitance is a direct measure for the degree of filling by the electrically conducting fluid component. An embodiment of an X-ray filter to be used in an X-ray apparatus of the invention is characterized in that the electric measuring means comprise an electric circuit with an AC source for providing an AC current to the first and second electrodes of individual filter elements and also comprising an AC detector to measure an AC signal induced by said current within individual filter elements. In this way in order to sense the level of the electrically conducting fluid component the electric measuring means comprising a simple electrical circuit are proposed to measure the electric capacitance.
In order to prevent the AC current from interfering with voltage supply means of the first electric potential a further embodiment of the filter according to the invention is characterized in that the electric measuring means comprise a further circuit element connected in series with the respective electric capacitance to form a voltage divider. A preferred embodiment for the circuit element, but not limited to it, is an additional electric capacitance.
Also, in order to prevent an electric interference between the first electric potential and an AC path, a fourth embodiment of the filter according to the invention is characterized in that the voltage supply means comprise a high impedance for supplying the first electric potential to individual filter elements.
In many radiodiagnostic applications it may be desirable to have a possibility of grey values per filter element, instead of working with filter elements which are either empty or full. One of the possible solutions for that problem is to divide the first electrode into a stack of electrode segments in the X-ray propagation direction. The control of the filter element filling in this case is achieved by the proper switching of the xe2x80x98fillxe2x80x99 voltage between the segments. A fifth embodiment of the filter according to the invention is characterized in that the first electrodes of individual filter elements comprise a stack of segments in the X-ray propagation direction, wherein the neighbouring segments are mutually electrically isolated, the first electric potential being switchable between the segments by the voltage supply means to alter the length of the column of the X-ray absorbing fluid in the X-ray propagation direction, the amplitude of the AC signal being a function of time, control means being provided for controlling the switching of the first electric potential between the segments in dependence of the behavior of the AC signal with time.
This embodiment is based on the insight that during the filling of a segment of the filter element by the electrically conducting fluid component, the electric capacitance per filter element is an increasing function with time. When a corresponding segment is filled and no xe2x80x98fillxe2x80x99 voltage is applied to the following segment, the electric capacitance reaches the saturation region. This region can be used as a trigger to set the xe2x80x98fillxe2x80x99 voltage to the next segment, if the fluid propagation within the inner volume of the filter element is desired. For that purpose, a sixth embodiment of the filter according to the invention is characterized in that the control means are conceived to switch the first electric potential between the segments triggered by the AC signal reaching a saturation region. The switching of the xe2x80x98fillxe2x80x99 voltage between the consequent segments is a time critical procedure, as the meniscus of the electrically conducting liquid must be situated in the vicinity of the following segment in order to feel the pulling force of the electrowetting power of the next segment. The saturation region of the electric capacitance curve indicates that the meniscus has arrived at the vicinity of the next segment and, therefore, the switching can be enabled.